The present invention relates to an air/fuel (A/F) ratio sensor which detects the A/F ratio of the feed to an internal combustion engine and other combustors on the basis of the concentration of oxygen in the exhaust from such combustors.
Various A/F ratio detectors are used to detect the A/F ratio of an air-fuel mixture feed into internal combustion engines and other combustors on the basis of the concentration of oxygen in the exhaust gas. The device shown in Unexamined Published Japanese patent application No. 178354/1984 comprises two elements each having porous electrodes formed on opposite sides of a tabular oxygen ion-conductive solid electrolyte and which are spaced from each other in a face-to-face relationship by a gas compartment, or the gap where the diffusion of the exhaust gas is limited; one of the elements is used as an oxygen pump for pumping oxygen out of the gap between the two elements, and the other element is used as an oxygen concentration electrochemical cell which produces a voltage in accordance with the difference in oxygen concentration between said gap and the ambient gas atmosphere to be analyzed. This device is so designed that it is capable of detecting an A/F ratio associated signal at least in the fuel-lean region.
It has, however, been found that this type of A/F ratio sensor, which is principally designed to issue a signal in the fuel-lean region where the exhaust gas contains a large amount of residual oxygen, produces a similar signal in the fuel-rich region where the exhaust gas contains a very small amount of residual oxygen because it reacts with other chemical species in the exhaust gas, such as CO, CO.sub.2 and H.sub.2 O. In other words, one signal from the A/F ratio sensor is associated with two different values of A/F ratio. Therefore, this A/F ratio sensor cannot be used for the purposes of A/F ratio control unless it is known definitely whether the combustor is operating in the fuel-lean or fuel-rich region.
According to one method that has been proposed for meeting this need, the atmospheric air is introduced such that it contacts the side of the oxygen concentration electrochemical cell which does not face the oxygen pump element, thereby preventing a detected signal from being inverted in the vicinity of the stoichiometric value of A/F ratio. However, in order to introduce the atmospheric air such that it contacts one side of the oxygen concentration electrochemical cell, the A/F ratio sensor must be open to the air and this in turn requires rendering the sensor waterproof by employing a complicated construction.
In order to eliminate this problem, one modification has recently been proposed; according to this proposal, instead of introducing the atmospheric air into the sensor, oxygen is generated at an internal reference oxygen source provided on one side of the oxygen concentration electrochemical cell element, and part of the evolved oxygen is caused to leak into the ambient exhaust gas or into the gas compartment through a leakage resisting portion so that the oxygen partial pressure in the internal reference oxygen source is maintained constant, thereby obviating the need for providing the atmospheric reference (see Japanese patent application Nos. 137586/1985 and 214004/1985).
When a predetermined amount of current is permitted to flow into the oxygen concentration electrochemical cell element of the A/F ratio sensor of the type described above, the oxygen in the gas compartment flows into the internal reference oxygen source and part of the oxygen inflow leaks to the outside through the leakage resisting portion so as to maintain a constant level of oxygen partial pressure in the internal reference oxygen source. This provides the results which are the same as those obtained by introducing the atmospheric air into the sensor and precise A/F ratio detection can be achieved without introduction of the atmospheric air.
Therefore, the A/F ratio sensor described in Japanese patent application Nos. 137586/1985 and 21 4004/1985 enables the concentration of oxygen in the exhaust gas to be detected as precisely as when the atmospheric air is introduced into the sensor. However, this sensor has detection characteristics which may be depicted as shown in FIG. 10 and the gradient differs so greatly between the fuel-lean and fuel-rich regions of A/F ratio that the precision of detection in the fuel-lean region is not as high as in the fuel-rich region. FIG. 10 depicts the change in pump current, Ip, which occurs when Ip flowing through the oxygen pump element is controlled such that a constant voltage will develop across the oxygen concentration electrochemical cell element. As shown in FIG. 10, the gradient of Ip in the fuel-rich region is about three times as large as the gradient in the fuel-lean region and this contributes to a reduced precision of detection in the fuel-lean region.
The above-described problem is due to the fact that non-oxygen gases such as CO and H.sub.2 have higher rates of diffusion than oxygen gas. Stated more specifically, in the fuel-rich region where a relatively small amount of oxygen is present in the exhaust gas, the sensor is so operated that the oxygen partial pressure in the gas compartment is held constant by means of the oxygen pump element which reacts with CO, CO.sub.2, H.sub.2 O and other oxygen-containing components in the exhaust gas to produce oxygen which then is pumped into the gas compartment. However, in the gas compartment, H.sub.2, CO and other rapidly diffusing gases will react with the oxygen that has been pumped into that compartment and the oxygen partial pressure in it will become lowered. In order to compensate for this pressure drop, the oxygen pump element is required to pump in a correspondingly larger amount of oxygen and this leads to an increased amount of pump current which flows through the pump element.
Also, the above-described sensor suffers from another problem. Namely, when it is used in exhaust gas of a motor vehicle, a deposit is adhered onto the gas diffusion limiting portion, resulting in clogging or plugging. This would lead to a defect such that there is an error in an output value representative of the time-basis change in gas diffusion control, i.e., the air/fuel ratio.